I. Field of the Invention
The present invention relates to a crankshaft assembly of the type used in internal combustion engines.
II. Description of Related Art
There are many types of previously known crankshafts that are used in internal combustion engines. These crankshafts typically include an elongated shaft adapted to rotate about its longitudinal axis. The shaft includes a radially offset portion between the ends of the shaft, and this offset portion has an axis parallel to, but radially spaced from, the axis of the shaft and this offset portion forms a crankpin. A pair of counterweights are secured to the shaft so that one counterweight is provided at each end of the radially offset portion of the shaft. These counterweights include a portion which extends in a direction diametrically opposed from the radially offset portion of the shaft.
An elongated connecting rod includes an annular bearing support at one end. This annular bearing support is positioned around the crankpin while the opposite end of the connecting rod is secured to a piston by a piston pin.
There have been a number of previously known methods for manufacturing these previously known crankshafts. In one prior method, the entire crankshaft together with the counterweights is formed as a one-piece forging and this forging is then machined to form not only the main bearing surfaces for the crankshaft, but also the bearing surface on the crankpin. In order to connect the connecting rod to the crankpin, the connecting rod bearing support typically comprised a two-part construction. The first part included a semi-circular bearing support which was positioned around the bearing crankpin. A semi-circular connecting rod cap was then also positioned around the crankpin so that the crankpin was sandwiched in between the connecting rod and the cap. The cap was then secured to the connecting rod by bolts or other fasteners.
These previously known forged crankshafts are advantageous since the entire crankshaft including the crankpin is of a one-piece construction. As such, the forged crankshafts exhibit high strength and failure of the crankshaft through breakage of the crankshaft is a rare phenomenon. A still further advantage of these previously known crankshafts is that the crankshaft is serviceable, i.e. in the event of damage to the connecting rod, the connecting rod can be removed and replaced. There are, however, several disadvantages of these previously known forged crankshafts.
One disadvantage of these previously known forged crankshafts is that the crankshafts are relatively expensive to manufacture. This relatively high cost of manufacture for the forged crankshafts results not only from the cost of preparing the forged crankshaft, but also the subsequent machining of the forging to form the main bearing surfaces of the crankshaft as well as the bearing surface on the crankpin.
A still further disadvantage of these previously known forged crankshafts is that, since the connecting rod is secured to the crankpin by threaded fasteners extending through the cap, failure of the connecting rod and/or the connection of the connecting rod with the crankpin can occur. Such failure typically occurs by failure of the connection of the connecting rod cap to the connecting rod.
There have, however, been previously known multi-piece connecting rods in which the shaft, crankpin and counterweights are all formed as separate components. The separate components are then secured together, for example by pressing these components together, in order to form the crankshaft assembly. Typically, the crankpin and shaft are formed of a high strength steel whereas the counterweights are formed of a less expensive material, such as powdered metal.
In order to secure the components together to form the overall crankshaft assembly, the counterweights are typically press fit onto the crankpin as well as to the shaft.
One advantage of these previously known multi-piece crankshafts is that the connecting rod can be attached to the crankpin before the other components of the crankshaft assembly, i.e. the counterweights and shaft, are attached together. Consequently, with multi-piece crankshafts, the annular bearing support of the connecting rod can be of a one-piece construction which is merely positioned over the crankpin prior to securing the other components of the crankshaft assembly together. Utilization of a one-piece bearing support on the connecting rod thus eliminates not only the two-piece connecting rod and bearing cap assembly used with forged crankshaft assemblies, but also the threaded fasteners for securing the cap to the connecting rod. This, in turn, results in a lower cost construction for the connecting rod but also in which the connection between the connecting rod and the crankpin exhibits higher strength than the previously known two-part connecting rod assemblies due to its one-piece construction.
One disadvantage of these previously known multi-piece crankshaft assemblies, however, is that the overall crankshaft assembly exhibits a much lower strength against torsional forces and the like than the previously known forged crankshafts. In practice, the connection between the various components of the multi-piece connecting rod can become loose which may result in overall failure of the crankshaft assembly.
There has, however, been one previously known crankshaft assembly in which the crankshaft, together with the crankpin and counterweight, was of a one-piece construction. In this previously known crankshaft assembly, the crankshaft included only one single counterweight at one end of the crankpin. A one-piece connecting rod having a one-piece annular bearing support was then positioned over the end of the crankshaft opposite from the counterweight and moved until the annular bearing support of the connecting rod was positioned around the crankpin. The opening of the annular bearing support of the connecting rod was usually larger than the outside diameter of the crankpin in order to enable the connecting rod bearing support to be slid from one end of the crankshaft and onto the crankpin. However, when positioned on the crankpin, bearings, such as needle bearings, were inserted in between the connecting rod bearing support and the crankpin thus completing the rotary connection between the connecting rod and the crankpin.
This previously known crankshaft advantageously enjoyed the high strength of the previously known forged crankshaft assemblies as well as the advantage of the one-piece connecting rod bearing support of the previously known multi-piece crankshaft assemblies. However, a primary disadvantage of this previously known crankshaft is that the crankshaft could only include a single counterweight at one end of the crankpin. Otherwise, it would not be possible to slide the connecting rod bearing support over the end of the crankshaft and onto the crankpin.
The present invention provides a crankshaft assembly which overcomes all of the above-mentioned disadvantages of the previously known crankshaft assemblies.
In brief, the crankshaft assembly of the present invention comprises an elongated one-piece shaft adapted to rotate about its longitudinal axis. This shaft has a radially offset portion between the ends of the shaft, and this radially offset portion forms the crankpin. As such, the axis of the crankpin is parallel to, but spaced from, the axis of the shaft.
An elongated connecting rod has a one-piece annular bearing support at one end of the rod. This annular bearing support is positioned around the crankpin by sliding the bearing support over one end of the shaft and onto the crankpin. Once the annular bearing support is positioned around the crankpin, a bearing assembly is inserted between the connecting rod bearing support and the crankpin in order to complete the rotatable connection between the connecting rod and the crankpin.
Although any conventional bearing assembly can be used, in the preferred embodiment of the invention, two half bearing shells are inserted in between the connecting rod bearing support and the crankpin. A pair of counterweights are secured to the crankshaft so that one counterweight is positioned at one end of the crankpin while the second counterweight is positioned at the other end of the crankpin. One counterweight is optionally secured to the shaft before the connecting rod bearing support is positioned over the crankpin whereas the second counterweight is secured to the shaft after the connecting rod bearing support is rotatably mounted to the crankpin by the bearing assembly. Optionally, however, both counterweights are secured to the shaft after the connecting rod is rotatably mounted to the crankpin.
Preferably, the first counterweight is formed by casting liquid metal, such as liquid zinc, onto the shaft. The second counterweight is preferably preformed in any conventional fashion, such as by a powdered metal component, and then press fit onto the shaft.
Since the shaft together with the pin is of a one-piece construction, preferably of high strength steel, the crankshaft assembly of the present invention enjoys the overall strength of the previously known forged crankshafts. Similarly, since the connecting rod is rotatably connected to the crankpin before at least one, and perhaps both, counterweights are secured to the shaft, a one-piece connecting rod with a one-piece annular bearing support can be utilized thus achieving the advantages of the previously known multi-piece crankshafts.
In a further embodiment of the present invention, a center guide assembly is provided between the connecting rod bearing support and the crankpin to limit movement of the connecting rod relative to the crankpin in the direction of the axis of the crankpin. This construction in turn enables both counterweights to be cast onto the crankshaft after the connecting rod is positioned over the crankpin.
In a still further embodiment, the crankshaft, crankpin and one counterweight are formed as a casting. A one-piece connecting rod is then slid onto the crankpin and a second counterweight is molded to the crankshaft on the side of the crankpin opposite from the cast counterweight.